When a hand-held scanner is used, which reads an original image by being freely scanned by hands on the surface of an original, it usually functions such that scanning positions of an image sensor on the original are detected sequentially and image data is stored into an image memory on the basis of the scanning positions being detected.
An example of a method of detecting scanning positions of an image sensor on an original is disclosed in U. S. Pat. No. 4,260,979. This method uses a sheet with a reference grid printed on it as an auxiliary equipment. By scanning the surface of this sheet placed on top of an original with a photosensing element which is integrated with an image sensor for detecting the reference grid, a position of image data against the reference grid is calculated from an output of the photosensing element.
Another method is disclosed, for example, in U. S. Pat. No. 4,581,761, which uses a tablet as an auxiliary equipment. By scanning on the surface of an original placed on top of the tablet with a detection coil which is integrated with an image sensor, a coordinate position on the tablet is detected from an output of the detection coil, and a position of the image data is calculated based on the coordinate position being detected.
Yet another method is disclosed, for example, in Laid-open Japanese Patent Application No. (Tokkai Sho) 62-15964, in which an encoder is installed with a wheel which moves together with an image sensor generating a pulse in accordance with the wheel rotation. A position of the image sensor is detected from an output of the encoder, and a position of image data is calculated on the basis of the detected position.
When an image processing apparatus which applies one of these conventional detection method is used, a coordinate position of image data is calculated based on a scanning position being detected and stored into a predetermined address of an image memory. As a result, this apparatus can be prevented from being influenced by unintentional hand movement or meandering caused by a hand-held scanning, so that image data being stored as storage data in the image memory does not show any distortion.
However, the conventional system mentioned above has the following problems.
When auxiliary equipment, such as a sheet with a reference grid printed on it, or a tablet is used, it is necessary to process with accuracy and to install an expensive adjustment circuit for enhancing accuracy of a scanning position being detected and reducing an absolute position error, so that the cost usually becomes high. In addition, when auxiliary equipment with low accuracy is used, the scanning position being detected shows a large degree of absolute position error.
On the other hand, when an encoder installed with a wheel, which moves together with an image sensor generating pulses in accordance with a wheel rotation is used, a scanning position is calculated by counting pulses of the encoder which is detected sequentially with a starting position of scanning as a reference point. As a result, a large degree of cumulative position error occurs due to machine accuracy and slipping of the wheel etc.
When an absolute position error or a cumulative position error of a scanning position being detected reaches a large degree, a large position error arises with a normal scanning position on an original which is actually scanned by an image sensor (this position error is hereinafter referred to as a "position error").
When a coordinate position of image data is calculated based on this scanning position having a position error and is then stored into a predetermined address in an image memory, a problem arises that images which are reproduced on the image memory show great distortion. The distortion of reproduction images caused by this position error will be explained with reference to FIGS. 19 (b) to 19 (d).
FIG. 19 (b) shows a hand-held scanner 1004 which scans freely on the surface of an original 1000. As shown in FIG. 19 (c), a prior hand-held scanner which is capable of scanning only one direction can not read a large size area on the original 1000. However, since image data is stored on an image memory only with an area being read by a sensor for one direction, a connection gap does not occur. FIG. 19 (d) shows a case, in which a hand-held scanner which reads a large size area on the original 1000 by a free scanning. In this case, when image data is stored in an image memory based on a scanning position having a position error, connected parts of the image cause a position error, so that the image is distorted. In particular, when a scanner causing a cumulative position error is used, a position error becomes worse as a scanning area broadens. Thus, it is no longer possible to make out the image itself.
Next, problems related to storing image data into an image memory will be explained.
As shown in FIG. 22, when both a part 2002, where image data is stored (image data storage pixel), and a part 2003, where image data is not stored (hereinafter referred to as a "map missing area"), are present inside an image memory 2001, white lines stand out greatly in a solid black area, so that the image quality deteriorates considerably. Also, as shown in FIGS. 23 (a) and 23 (b), when this map missing area arises in a character area, white lines appear in a character, so the qualtiy of the character deteriorates considerably.